1. Field of the Invention
The present invention relates to an electronic device including a charge/discharge control circuit, which is used to charge a capacitor with minute electric power, and to supply charged power of the capacitor to a load when a voltage of the capacitor to be increased by the charge reaches a predetermined voltage.
2. Description of the Related Art
In recent years, there have been an increasing number of electronic devices which operate by using electric power generated by sunlight in one's surroundings, a body temperature of a human being, or the like. A solar cell is known as one that generates electric power by sunlight, and a thermoelectric conversion element is known as one that generates electric power by the body temperature of a human being. However, these generators are downsized for improving portability and cutting costs, and hence the generated power is minute. Therefore, there have been an increasing number of cases where the generated power is less than the consumption power of the electronic devices. In this case, a charge/discharge control circuit is used to charge a capacitor with generated power once, and to supply charged power of the capacitor to the electronic device when enough electric power is charged for operating the electronic device for a predetermined period of time.
FIG. 4 illustrates a conventional electronic device including a charge/discharge control circuit. The electronic device includes a power source 401, a capacitor 405, a control circuit 406 for controlling charge/discharge, and an electronic device body circuit 407 serving as a load. The power source 401 includes a thermoelectric conversion element 402, a booster circuit 403, and a Schottky diode 404.
The thermoelectric conversion element 402 converts a temperature, such as a body temperature and an outside air temperature, into generated power and outputs the generated power. The generated power output from the thermoelectric conversion element 402 has a voltage less than an operation voltage of the electronic device body circuit 407. Therefore, in the booster circuit 403, the generated power is converted into boost power having a voltage equal to or more than the operation voltage of the electronic device body circuit 407. The boost power is output from the booster circuit 403 via the Schottky diode 404 for preventing reverse flow, and charges the capacitor 405. The control circuit 406 includes a switching element and a voltage detection circuit, and a hysteresis circuit and a delay circuit, or a latch circuit, a timer circuit, and a delay circuit. The voltage detection circuit monitors a voltage of the capacitor 405, and hence monitors a charge amount of the capacitor 405, thereby detecting that the charge amount of the capacitor 405 has reached electric power high enough for operating the electronic device body circuit 407 for a predetermined period of time.
When the control circuit 406 detects the above-mentioned state, the switching element is turned ON, thereby supplying charged power of the capacitor 405 to the electronic device body circuit 407. When the control circuit 406 includes the hysteresis circuit and the delay circuit, a detection voltage is provided with hysteresis, and the switching element is turned ON after a delay period of the delay circuit. Then, the charged power of the capacitor 405 is supplied to the electronic device body circuit 407 until the voltage of the capacitor 405 becomes a hysteresis voltage. When the control circuit 406 includes the latch circuit, the timer circuit, and the delay circuit, after detecting the voltage of the capacitor 405, inversion of the latch circuit and operation of the timer circuit are performed during a delay time of the delay circuit. Then, the switching element is turned ON after the delay time of the delay circuit. Then, until the latch circuit is reset by the timer circuit, the charged power of the capacitor 405 is supplied to the electronic device body circuit 407. When the charged power of the capacitor 405 is supplied to the electronic device body circuit 407, the electronic device body circuit 407 operates (see, for example, Japanese Patent Application Laid-open No. Hei 11-288319).
In the above-mentioned conventional electronic device including a charge/discharge control circuit, the voltage of the capacitor is monitored, and, when it is detected that the voltage of the capacitor has become a predetermined voltage, the charged power of the capacitor is supplied to the electronic device body circuit serving as a load. In this configuration, during the delay time, which is after it is detected that the voltage of the capacitor has become the predetermined voltage and until the switching element is turned ON, it is required to determine the inversion of the latch circuit or the operation of the hysteresis circuit before the switching element is turned ON. In order to invert the latch circuit or operate the hysteresis circuit, a potential difference of several mV or more is necessary after the start of the operation and until the completion thereof During the above-mentioned delay time, the voltage of the capacitor needs to increase to this potential difference or more. When the switching element starts being turned ON in a state where the voltage of the capacitor has not reached this voltage or more, the electric power to be supplied to the electronic device body circuit gradually increases. When this electric power becomes equal to the charged power, the voltage of the capacitor stops increasing. Then, when the voltage of the capacitor stops increasing, the operation of the control circuit 406 stops while the operation of the latch circuit or the hysteresis circuit and an ON resistance of the switching element are left in an indefinite state. The charged power keeps being consumed by the electronic device. As a matter of course, as compared to the electric power necessary for the operation of the electronic device body circuit, the charged power in this case is much smaller, and hence the electronic device body circuit cannot perform a desired operation.
As described above, in the conventional electronic device including a charge/discharge control circuit, there has been a problem that desired electric power cannot be supplied to a load in a case where the charged power is minute and the voltage increase rate of the capacitor, which increases by charge, is low. Further, the generated power is decreased due to downsizing of the generator, and a consumption current is increased due to improvement of functions and performance in the electronic device operated by the generated power, and hence there have been an increasing number of cases where a capacitance value is increased, with the result that the above-mentioned problem is liable to occur.